Nested conditional relations (NCR) model and algebra

ABSTRACT

A method and system for providing data integration of multiple data stores with diverse formats. The data integration engine accepts queries using a standard query language such as XML-QL, executes those queries against the multiple data stores, and returns the results. The data stores may include relational databases, hierarchical databases, file systems, application data available via APIs, and so on. A query may reference data that resides in different data stores. The data integration engine allows operations such as joins across multiple data stores.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/222,070 filed Aug. 1, 2000 and is related to U.S. patent application Ser. No. 09/718,228 filed Nov. 21, 2000, U.S. patent application Ser. No. 09/517,131 filed Mar. 2, 2000 and U.S. patent application Ser. No. 09/517,468 filed Mar. 2, 2000, which are hereby incorporated by reference.

BACKGROUND

The described technology relates generally to accessing data and particularly to accessing data from data sources with diverse formats.

Large organizations may have their digital data stored in various data stores, such as databases and file systems, in diverse and incompatible formats. Different groups within the large organizations may have created their own data stores to meet the needs of the group. Each group would typically select its own type of data storage system and format to meet its particular needs. Traditionally, these data stores were created independently of any other data stores within the organization. As a result, the various data stores of an organization often contained duplicate and inconsistent data.

Recently, these large organizations have adopted standards such as the extensible markup language (“XML”) for representing data in a uniform format. The use of XML by each group within an organization increases the compatibility of the data stores. It is, however, difficult for organizations to provide an XML interface to each of its existing data stores. The organizations would need to expend considerable resources to provide a mapping between their existing data stores or other sources of data and the XML formats.

It would be desirable to have a system that would facilitate the integrating of data stores with incompatible formats.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the schema of this XML document.

FIG. 2 represents a JoinIn graph (JIG) for the match expression of Table 8.

FIG. 3 is a block diagram illustrating the overall organization of an execution program generated by the data integration engine.

FIG. 4 is a block diagram illustrating the function to generate an execution program.

FIG. 5 is a flow diagram illustrating processing of the generate extract program function in one embodiment.

FIG. 6 is a flow diagram illustrating the processing of the generate extract plan function in one embodiment.

FIG. 7 is a flow diagram illustrating processing of the match expression function in one embodiment.

FIG. 8 is a flow diagram illustrating the processing of the create JoinIn graph function in one embodiment.

FIG. 9 is a flow diagram illustrating processing of the generate JoinIn graph into one embodiment.

FIG. 10 illustrates the tables of the data store.

FIG. 11 illustrates the results of the sorted outer union for the tables of FIG. 10.

FIG. 12 illustrates the SQL query for each of the tables of FIG. 10 paid to generate the sorted outer union.

FIG. 13 is a flow diagram illustrating the processing of generating a sorted outer union.

FIG. 14 is a flow diagram illustrating processing of a generate SQL query function in one embodiment.

FIG. 15 is a block diagram illustrating an extract program.

FIG. 16 is a flow diagram that illustrates code of a join node of an extract program in one embodiment.

FIG. 17 illustrates the output of the nodes of the extraction plan to FIG. 15.

FIG. 18 illustrates a final NCR structure.

FIG. 19 illustrates the Correspondence Tree.

FIGS. 20-25 illustrate normalization.

DETAILED DESCRIPTION

A method and system for providing data integration of multiple data stores with diverse formats is provided. In one embodiment, the data integration engine accepts queries using a standard query language such as XML-QL, executes those queries against the multiple data stores, and returns the results. The data stores may include relational databases, hierarchical databases, file systems, application data available via APIs, and so on. A query may reference data that resides in different data stores. The data integration engine allows operations such as joins across multiple data stores. In one embodiment, the data integration engine uses XML as the data model in which the data from the various data stores is represented. The data integration engine processes a query by parsing the query into an internal representation, compiling and optimizing the internal representation into a physical execution representation, and then executing the execution representation. By providing a uniform and data model, the data integration engine allows access to data stores in diverse formats.

In one embodiment, the data integration engine executes a query on a data store by first providing a mapping of the data store format into an XML format. The query for the data store is based on XML format. The data integration engine upon receiving a query, generates a native query for the data store from the received query using the provided mapping. The data integration engine then executes the native query to generate data in a native format needed to generate the results of the received query. The data integration engine then converts the data in the native format into data in a format referred to as nested conditional relations (“NCR”). The data integration engine then applies various operators (e.g., joins and unions) to the data in NCR format to generate the query results in an NCR format. The data integration engine then converts the results in the NCR format into an XML format. In this way, the integration engine can provide access to various data sources in different formats.

A nested conditional relation is a table in which each row may have a different schema and each column is either a primitive type or a nested NCR. The schema of each row in an NCR is indicated by a tag, which can be considered to be the zero column of the row. For example, certain rows of the table may represent employees of a company and have columns named “first name,” “last name,” “phone number,” and so on. Other rows in the table may represent departments within the company and have columns named “department name,” “department head,” and so on. The tag for a row indicates whether the row is an employee or a department row. A column for a certain type of row may itself contain a nested conditional relation. For example, an employee row may include a column named “skills” that contains a table with sub-rows containing information relating to computer skills and accounting skills of the employee. The table may itself be a nested conditional relation in that each sub-row may include a tag indicating whether the row represents a computer skill or an accounting skill. The nesting of nested conditional relations may occur to an arbitrary level. The NCR format is described below in detail.

The following example illustrates a data store, a mapping for the data store, a query, an LMatch representation for the query, a JoinIn graph for the query, and an SQL query used to retrieve the data from the data source. Tables 1-3 illustrate an example of data that is stored in a data store such as a relational database. The relational database contains three tables: DEPARTMENTS table, EMPLOYEES table, and BUILDINGSDOCS table.

TABLE 1 DEPARTMENTS Name Contact Finance E1247 Engineering E3214

TABLE 2 EMPLOYEES Of- Man- ID Fname Lname Dept Bldg fice ager E0764 Bobby Darrows Finance B 102 E1247 E0334 Alice LeGlass Finance B 103 E1247 E1247 David Winston Finance B 110 NULL E3214 David McKinzie Engineering L NULL E1153 E0868 Misha Niev Engineering L  15 E1153 E0012 David Herford Engineering M 332 E1153 E1153 Charlotte Burton Engineering M 330 E0124 E0124 David Wong Engineering L  12 NULL

TABLE 3 BUILDINGSDOCS Building Office Phone MaintContact B 102 x1102 E0764 B 103 x1103 E0764 B 110 x1110 E0764 L lobby x0001 E3214 L  12 x0120 E3214 L  15 x0150 E3214 M 330 x2330 E3214 M 332 x2332 E3214

The DEPARTMENTS table contains one row for each department of an organization. As illustrated by Table 1, the organization has a finance and an engineering department. The DEPARTMENTS table contains two columns: name and contact. The name column contains the name of the department, and the contact column contains the employee identifier of the contact person for the department. For example, the first row of the table indicates that the department is “finance” and that the contact employee is “E1247.” The EMPLOYEES table contains a row for each employee in the organization. Each row includes seven is columns: ID, Fname, Lname, Dept, Bldg, Office, and Manager. The ID column uniquely identifies the employee, the Fname column contains the first name of the employee, the Lname column contains the last name of the employee, the Dept column identifies the employee's department, the Bldg column identifies the building in which the employee is located, the Office column identifies the employee's office within the building, and the Manager column identifies the employee's manager. The Dept column contains one of the values from the Name column of the DEPARTMENTS table. The BUILDINGSDOCS table contains a row for each office within each building of the organization. The BUILDINGSDOCS table contains four columns: Building, Office, Phone, and MaintContact. The Building column identifies a building, the Office column identifies an office within the building, the Phone column contains the phone number associated with that office, and the MaintContact column identifies the employee who is the maintenance contact for the office. The combination of the Building and Office columns uniquely identifies each row. The Bldg and Office columns of the EMPLOYEES table identifies a row within the BUILDINGSDOCS table.

Table 4 is an example of data stored as an XML document.

TABLE 4 <deptlist> <deptname=“Finance”> <employee> <name><first>Bobby</first><last>Darrows</last></name> <office phone=“x1102”/> </employee> <employee> <name><first>Alice</first><last>LeGlass</last></name> <office phone=“x1103”/> </employee> ... </dept> <dept name=“Engineering”> <employee> <name><first>David</first><last>McKinzie</last></name> </employee> <employee> <name><first>Misha</first><last>Niev</last></name> <office phone=“x0150”/> </employee> ... </dept> </deptlist>

The XML document includes the root element <deptlist> that has a name attribute and that contains a <dept> element corresponding to each department within an organization. Each <dept> element contains an <employee> element for each employee within the department. Each <employee> element contains a <name> element and optionally an <office> element. The <name> element includes a <first> element and <last> element. The <office> element includes a phone attribute. The schema of an XML document may be represented by an XML data type definition (“DTD”) of the document. FIG. 1 illustrates the schema of this XML document. As this figure illustrates, the schema is specified as a tree-like hierarchy with the nodes of the tree having parent-child relationships. For example, node 104 is the parent of nodes 105 and 108, which are children of node 104. Node 101 corresponds to the <deptlist> element and has one child node 102, which corresponds to the <dept> element. Node 102 has two child nodes, 103 and 104. Node 104 corresponds to the name attribute of the <dept> element and node 104 corresponds to the <employee> element. Node 104 has two child nodes 105 and 108. Node 105 corresponds to the <name> element and has two child nodes 106 and 107. Node 106 corresponds to the <first> element, and node 107 corresponds to the <last> element. Node 108 corresponds to the <office> element and has one child node 109, which corresponds to the phone attribute.

The mapping technique is particularly useful in situations where a legacy database, such as the example database of Tables 1-3, is to be accessed using queries designed for XML data, such as the example of Table 4. The XML schema may be previously defined and many different applications for accessing data based on that XML schema may have also been defined. For example, one such application may be a query of the data. An example query for semi-structured data may be an XML transform that is designed to input data in XML format and output a subset of the data in XML format. For example, a query for the database of Tables 1-3 may be a request to list the ID of each employee in the “Finance” department. The subset of that data that is output corresponds to the results of the query represented by the XSL transform. One skilled in the art would appreciate that queries can be represented in other formats such as XML-QL. When a legacy database is to be accessed, the data is not stored using XML format. Thus, in one embodiment, a query system inputs a semi-structured query and uses a mapping table to generate a structured query, such as an SQL query, that is appropriate for accessing the legacy database. The mapping technique for generating that mapping table is described in the following.

Table 5 is a portion of the mapping table generated in accordance with the mapping technique that maps the XML schema of Table 4 to the legacy database of Tables 1-3.

TABLE 5 Row ParentName A/E ChildName Table Pkey Ckey 1 deptlist E dept DEPARTMENTS Name 2 dept A name DEPARTMENTS Name Name 3 dept E employee EMPLOYEES Dept ID 4 employee E name EMPLOYEES ID ID 5 name E first EMPLOYEES ID Fname 6 name E last EMPLOYEES ID Lname 7 employee E office EMPLOYEES ID {Bldg,Office} 8 office A phone BUILDINGSDOCS {Building,Office} phone

The mapping table contains one row for each parent-child relationship of the XML schema. The mapping is further described in U.S. patent application entitled “Method and Apparatus for Storing Semi-Structured Data in a Structured Manner.” As shown in FIG. 1, the XML schema defines eight parent-child relationships such as the relationship between node 102 and node 104. Thus, the mapping table contains eight rows. Each row uniquely identifies a parent-child relationship using the ParentName and ChildName columns. For example, the parent-child relationship of node 102 and node 104 is represented by row 3 as indicated by the ParentName of “dept” and the ChildName of “employee.” Each row maps the parent-child relationship to the table in the legacy database that corresponds to that relationship. In the example of row 3, the Table column indicates that the “dept-employee” relationship maps to the EMPLOYEES table. The query system could use only the ParentName, ChildName, and Table columns of the mapping table to generate a structured query from a semi-structured query. For example, if the legacy database had used the same column names as defined by the elements of the XML schema (e.g., “employee” rather than “ID”), then only these three columns would be needed to generate the structured query. For example, if the semi-structured query requested an identifier of all employees within the finance department and the DEPARTMENTS table contained an “employee” column rather than an “ID” column, then the query system could input a semi-structured query with only these three columns and generate a structured query. In the more general case where the columns of the legacy database are arbitrarily named, the mapping table includes a parent key column (“PKey”) and a child key column (“CKey”). The parent key column contains the name of the column that identifies the parent of the parent-child relationship. The child key column contains the name of the column that identifies the child of the parent-child relationship. For example, in row 3, the parent is identified by the “dept” column and the child is identified by the “ID” column in the EMPLOYEES table. Thus, to generate the structured query to retrieve the ID of an employee within the finance department, the query that uses a select clause of EMPLOYEES.dept=“Finance” would be used. Table 5 also includes a column named “A/E” to indicate whether the row corresponds to an element within the semi-structured data or an attribute of an element with semi-structured data. As illustrated by rows 7 and 8, some of the parent and child keys actually consist of multiple columns that uniquely identify a row in the corresponding table. For example, the rows of the BUILDINGSDOCS table are uniquely identified by a combination of the Building and Office columns.

The query system maps the selections within the semi-structured query to selections within a structured query. The following illustrates the basic format of that mapping when the structured query is an SQL format.

SELECT {TABLE}.{CKEY} FROM {TABLE} WHERE {TABLE}.{PKEY} = pkey

The TABLE, CKEY, and PKEY parameters are replaced by the corresponding values from the row in the mapping table for the parent-child relationships specified by the selection. In other words, this query will find all the children given the key for the parent. The following illustrates the format of the mapping when the query represents the identification of the idea of all employees within the finance department.

SELECT EMPLOYEES.ID FROM EMPLOYEES WHERE EMPLOYEES.Dept = “Finance”

The query system also allows chaining of keys to effectively navigate through the hierarchy defined by the semi-structured data. The query system uses the joint concept of relationship databases to effect this chaining of keys. The following illustrates chaining:

 SELECT {TABLE2}.{CKEY2}  FROM {TABLE1}, {TABLE2}  WHERE {TABLE1}.{PKEY1} = pkey && {TABLE1}.{CKEY1}=  {TABLE2}.{PKEY2}

The TABLE1, PKEY1, and CKEY1 parameters are derived from the first parent-child relationship in the chain, and the TABLE2 , PKEY2, and CKEY2 parameters are derived from the second parent-child relationship in the chain. The child key associated with the first parent-child relationship matches the parent key associated with the second parent-child relationship. The following is an example of the chaining to identify the building for the employees of the finance department.

SELECT BUILDINGSDOCS.BUILDING FROM EMPLOYEES, BUILDINGSDOCS WHERE EMPLOYEES = “Finance” && EMPLOYEES.BLDG = BUILDINGDOCS.BUILDING && EMPLOYEES.OFFICE = BUILDINGDOCS.OFFICE

In one embodiment, the mapping table also contains the value rows corresponding to each leaf node, that is a node that is not a parent node. The leaf nodes of FIG. 1 are nodes 103, 106, 107, and 109. In one embodiment, each value row identifies an XML element or attribute, the table in the legacy database that contains an element, and the name of the column in the table that contains the value for that element or attribute. Table 6 illustrates the four value rows for the mapping associated with Tables 1-3 and Table 4.

TABLE 6 Row A/E Name Table Key Value  9 A name DEPARTMENTS Name Name 10 E first EMPLOYEES Fname FName 11 E last EMPLOYEES Lname LName 12 A phone BUILDINGSDOCS Phone Phone

The “A/E” column identifies whether the row is an attribute or element; the “Name” column identifies the name of the element and attributes; the “Table” column identifies the legacy table; the “Key” column identifies the key for that table; and the “Value” column identifies the name of the column where the value is stored.

Table 7 illustrates a query that is to be applied to the data of Tables 1-3. The query indicates to return the first and last names and phone number of each employee in the engineering department.

TABLE 7 WHERE <deptlist> <dept name=“Engineering”> <employee>  <name><first>$first</first><last>$last</last></name> <office phone=“$ph”/> </employee> </dept> </deptlist> CONSTRUCT <employee><name>$last, $first</name><phone>$ph</phone> </employee>

The data integration engine generates a “match expression” for a logical match operation (“LMatch”) for the query when compiling the query. The logical match operation supports operations for performing XML navigation. The match expression defines a tree of navigations. Each node of the tree indicates a navigation type (e.g., child, parent, or sibling), a navigation condition (e.g., a condition on the name of the child), whether the navigation is required, whether there should be a binding to the target of the navigation (i.e., a value returned with the specified name), and whether the result should be nested.

Table 8 illustrates a match expression for the XML of Table 4 for the query of Table 7. Each row of Table 8 represents a different navigation path. For example, the first row represents a navigation path from the root of the deplist element to its child element of the dept element and then to the name attribute of the dept element. The remaining rows represent different branches on the tree. For example, the second row represents the branch of root(deplist), child(dept), child(employee), child(name), and child(first). The symbols prefixed with “$” represent bindings.

TABLE 8 root(deptlist) child(dept) child(name,$auto1) child(employee) child(name) child(first, $first) child(last, $last) child(office) child(phone, $ph)

FIG. 2 represents a JoinIn graph (JIG) for the match expression of Table 8. The JoinIn graph is a data structure that facilitates the optimization of the query to be executed against the data store. This JIG indicates that the Departments, Employees, and Buildingdocs tables of the data store are to be joined together. This JIG also indicates the bindings (e.g., $first) and the join columns (e.g., Name and Dept). The format of the JIG is described below in detail. The JIG is generated from the match expression using the mapping. The data integration engine then generates the query to be executed. The following query is generated.

SELECT EMPLOYEES.Fname, EMPLOYEES.Lname, BUILDINGSDOCS.phone FROM DEPARTMENTS, EMPLOYEES, BUILDINGSDOCS WHERE DEPARTMENTS.Name = EMPLOYEES.Dept AND EMPLOYEES.Bldg = BUILDINGDOCS.Building AND EMPLOYEES.Office = BUILDINGSDOCS.Office AND DEPARTMENTS.NAME = “Engineering”

FIG. 3 is a block diagram illustrating the overall organization of an execution program generated by the data integration engine. An execution program consist of an extract program 310 and a construct program 320. A compiler of the data integration engine generates the execution program during a compilation phase. The extract program is a series of operations on a data extracted from the data sources. The extract program represents a graph of the operations. The leaf nodes 311 of the extract program represents a sorted outer union operation applied to the data stores 312. The compiler generates a query for each data store in the native query language of the data store to retrieve the results of the sorted outer union. The compiler generates the sorted outer union using the LMatch operation, JoinIn graph, and mapping. During execution of the extract program, the generated query is applied to each data store. The construct program accesses the root node 313 of the extract program which retrieves the results generated by the extract program. The construct program collects the data and formats it into an XML output. As discussed below in more detail, the output of each operation of the extract program is in a nested conditional relation format.

FIGS. 4-9 are flow diagrams illustrating processing of the compiler of the data integration engine in one embodiment. FIG. 4 is a block diagram illustrating a function to generate an execution program. The function first generates the extract program and then generates the construct program. In block 401, the function invokes a generate extract program function to generate an extract program for the specified query against the specified data stores. In block 402, the function invokes the generate construct program function to generate a construct program to generate the results from the extracted data.

FIG. 5 is a flow diagram illustrating processing of the generate extract program function in one embodiment. In block 501, the function generates an extract plan. In block 502, the function identifies fragments of the extract plan. A fragment of an extract plan are the set of operations that are applied to data derived from a single data source. Operations that apply to data from multiple data sources are grouped into one fragment. In block 503, the function optimizes the operations of the fragments and then returns.

FIG. 6 is a flow diagram illustrating the processing of the generate extract plan function in one embodiment. In block 601, the function receives the XML query. In block 602, the function generates a match expression for the logical match associated with the data store. In block 603, the function creates the JoinIn graph from the match expression using the mapping for the data store. In block 604, the function generates the native query from the JoinIn graph. The function indicates additional processing to generate the extract plan from the JoinIn graph. Blocks 602-604 illustrate the generation of the native query for the sorted outer union of the leaf nodes of the extract plan. The ellipses indicate other processing performed by the function. The function then returns.

FIG. 7 is a flow diagram illustrating processing of the match expression function in one embodiment. This function is passed an XML node representing the data store and returns the match expression. This function is recursively invoked for each child node of the passed XML node. In block 701, the function initializes the sub-tree to the XML node. In block 702-705, the function loops creating a match expression for each child node. In a block 702, the function selects the next child node of the XML node. In decision block 703, if all the child nodes have already been selected, then the function returns, else the function continues at block 704. In block 704, the function recursively invokes the create match expression function passing the child node and receiving a child sub-tree in return. In block 705, the function adds the child sub-tree to the sub-tree and then loops to block 702 to select the next child.

FIG. 8 is a flow diagram illustrating the processing of the create JoinIn graph function in one embodiment. In block 801, the function invokes the generate JoinIn graph passing the match expression and receiving the JoinIn graph in return. In block 802, the function merges nodes of the JoinIn graph. In block 803, the function processes merging of adjoining nodes of the JoinIn graph and then returns.

FIG. 9 is a flow diagram illustrating processing of the generate JoinIn graph function into one embodiment. This function is passed a match expression and returns a JoinIn graph. The function is recursively invoked for each child node of the passed match expression. In block 901, the function sets the JoinIn graph to a node corresponding to the root of the match expression. The function retrieves the mapping rows that can further the path from the root. In block 902, the function selects the next child node of the match expression. In decision block 903, if all the children have already been selected, the function returns, else the function continues at block 904. In block 904, the function recursively invokes the generate JoinIn graph function passing the selected child node of the match expression and receiving a child JoinIn graph in return. In block 905, the function adds the child JoinIn graph to the JoinIn graph and then loops to block 902 to select the next match expression.

FIGS. 10-15 illustrate the generation of an SQL query for a sorted outer union node of an extract program. FIG. 10 illustrates the tables of the data store. The arrows between the tables illustrate joins between tables. For example, arrow 1001 represents a join between the third column of table 1.1 and the first column of table 2.1. FIG. 11 illustrates the results of the sorted outer union for the tables of FIG. 10. FIG. 12 illustrates the SQL query for each of the tables of FIG. 10 that are used to generate the sorted outer union.

FIG. 13 is a flow diagram illustrating the processing of a function to generate a sorted outer union. In block 1301, the function selects the next table of the source data store. In decision block 1302, if all the tables have already been selected, the function continues at block 1304, else the function continues at block 1303. In block 1303, the function invokes the generate SQL query for the selected table and then loops to block 1301 to select the next table. In block 1304, the function executes the generate SQL queries against the tables. In block 1305, the function aggregates of the result of the queries into a table. In block 1306, the function sorts the results and then returns.

FIG. 14 is a flow diagram illustrating processing of a generate SQL query function in one embodiment. In block 1401, the function outputs a select, from, and where clause for the query. In blocks 1402-1408, the function loops selecting each table in a join path of the data store. In block 1402, the function selects the next table in the path. In decision block 1403, if all the tables have already been selected, then the function returns, else the function continues at block 1404. In block 1404, the function adds the table to the from clause. In block 1405, the function adds the table to the where clause. In block 1406-1408, the function loops selecting each column of the selected table. In block 1406, the function selects the next column. In decision block 1407, if all the columns have already been selected, then the function loops to block 1402 to select the next table of the path, else the function updates the select clause with the column and then loops to block 1406 to select the next column. Columns of tables not in the selected path are set to null.

FIG. 15 is a block diagram illustrating an extract program. Each of the leaf nodes 1501-1505 represent SQL queries that are applied to a data source. Node 1506 represents a nesting of the results of nodes 1501 and 1502. Node 1507 represents a nesting of the results of nodes 1506 and 1503. Node 1508 represents a selection on the results of node 1507. Node 1509 represents a nesting of the results of nodes 1504 and 1505. Node 1510 represents a join of the results of nodes 1508 and 1509. Node 1511 represents a projection of the results of node 1510. Node 1512 represents the construct program that accesses the extract program.

FIG. 16 is a flow diagram that illustrates code of a join node of an extract program in one embodiment. In one embodiment, the processing of each node of extract program is performed a pipeline manner, that is each node returns only the data needed to satisfy the next request from the construct program. In decision block 1601, if the right node is a fully processed, then the function continues at block 1602, else the function continues at block 1605. In decision block 1602, if the left node of the join is fully processed, then the function returns, else the function continues at block 1603. In block 1603, the function retrieves at the next results from the left node. In block 1604, the function initializes the right node based on the results returned from the left node. In block 1605, the function retrieves the next results from the right node. In decision block 1606, if the results returned from the right node are contained in nested table, then the function returns an iterator for that table, else the function returns the results. The iterator for a table is an optimization that allows nodes higher in the extract program to retrieve subsequent rows of the nested table without having to invoke lower-level nodes in the extract program.

FIG. 17 illustrates the output of the nodes of the extraction plan to FIG. 15. When the construct program 1712 invokes root node 1711 of the extract program, that invocation is propagated down to the leaf nodes. The SQL query of node 1701 returns result 1713, and the SQL query of node 1702 returns result 1714. Node 1706 indicates to nest results of nodes 1701 and 1702. In this case, result 1714 is nested within result 1713 as indicated by result 1715. Node 1703 returns result 1716. Node 1707 nests result 1716 within result 1715. The subscript within node 1707 specifies a target for the nesting. In this case, the subscript 2 indicates to nest within the third column of result 1715. (Columns are identified starting with column 0.) Result 1717 represents the result of the nesting. Node 1708 represents selection on the result 1717. The target represented by subscript 2.1 indicates to select the third column and the first row within the third column. The result of the selection is result 1718. Results 1719-1723 illustrates the results of the other nodes of the extract program.

LMatch Operation

The LMatch operator performs navigation-based selection over XML input. The following example illustrates an XMLQL syntax fragment and the LMatch instance that is created to model it inside the compiler:

-   -   <a><b><c>$c</></></> ELEMENT_AS $a     -   LMatch($results, “self(a,$a)—child(b,--)—child(c,$c)”)

The “self(a,$a)—child(b,--)—child(c,$c)” is a match expression. In this example, the match expression is a tree with three nodes. The general structure of the XMLQL pattern is translated into an isomorphic pattern within the match expression. The XMLQL variables become “bindings” within the navigations. The LMatch operator is one of the logical operators of the internal language of the data integration engine. The LMatch operator is generally the “first” operator that is applied to input data and is responsible for converting XML input in to NCRs that are then further processed by the query engine. The LMatch operator is a logical operator only in that one of the actions of the Compiler is to convert LMatch operators into a data source-dependent form (e.g., SQL for relational databases, or QLL for QL-Lite data sources).

The LMatch operator defines a match against XML data. The pattern is defined by the “match expression,” which is a tree of navigation steps. Each navigation step describes a “movement” from a source element or attribute to a target element or attribute. The parameters of the navigation step that govern navigation are as following:

-   -   The type of movement or navigation (child, parent, descendant         etc.) The navigation types are based on XPath axes.     -   The name of the target element or attribute, which may be a wild         card.     -   Whether the target should be an element, an attribute, or         either.     -   Whether the navigation is optional or not.

LMatch matching is top-down on the tree of navigation steps. That is, the match begins at the root of both the XML document and the root of the match expression. Matches for the first navigation step are sought in the entire XML document. If the first navigation is a root navigation, then it matches the root of the XML document (where we interpret root to be the root element, not the document item, as defined in DOM). If the first navigation step is something other than root, it is as a navigation from the root.

Once a node or set of nodes have been identified for the first navigation, the algorithm proceeds recursively: given a matched node, attempt each of the child navigations from the navigation tree (where child here means “child in the navigation tree,” rather than child type node). Each attempted navigation will itself yield a new set of zero or more matches, which are then continued in the next level of the recursion, and so forth. While the recursion proceeds down the navigation tree, the navigations do not necessarily proceed “down” the XML tree; navigation types can move in arbitrary directions within the XML document (e.g., ancestor or preceding_sibling).

If an attempted navigation yields zero matches from some source node, then that navigation is said to have failed. If the navigation was not marked as optional=true, then the failure of the naviagation causes the source node to be “unmatched.” The following match expression illustrates the failure of a navigation;

-   -   self(a,$a)—child(b,$b)—child(c,$c)

The first navigation step may yield a single element <a>. The second step may yield a set of <b> elements, some of which contain <c> elements and some of which do not. When the final navigation is evaluated, it will, for some <b> elements, yield no results. If the navigation is optional (optional=true), then all the <b> elements are included in the result. If, however, the navigation is required (optional=false), then those <b> elements that contain no <c> elements are removed from the set of matches for child(b) from the root <a> element. The result contains only <b> elements that actually contain <c> elements. If no <b> elements remain after this process, then the failure propagates upward, “unmatching” the <a> elements (unless the child(b) navigation was optional).

The evaluation of an LMatch operator is a three stage process: first, match the pattern within the LMatch operator against some source of XML; second, connect columns in the LMatch pattern with their associated items in the information set of the XML source; and thirs, structure those connected columns (the extracted information) into an NCR as indicated by the nesting settings on individual navigations. That is, an LMatch operator specifies a structural pattern that is sought after in a document, specifies which parts of that pattern should be returned, and specifies how the returned parts should be organized. The output of an LMatch operator is an NCR that contains the returned parts, organized as specified.

The parameters of the LMatch that govern how results are constructed are these:

-   -   The set of columns returned. An NCR column “names” some piece of         information returned from an element or attribute node that has         been matched. There are several kinds of columns:         -   value: the contents of a simple element or attribute         -   subtree: the entire element (not applicable to attributes)         -   name: the name of the element or attribute         -   text: the text value of an element (used to extract text             from mixed-content elements; in a simple element it is             equivalent to ‘value’)         -   table: the table column gives a name to the entire set of             results when nested=true     -   Whether or not the results of the navigation should be nested.

Each navigation step may have one or more of the column types present. The type of the column is derived from the type of the corresponding contents of the XML document (except for the table column).

These columns are structured into an NCR based on the nested flag and the table column: If nested true, then the table column was specified, and the navigation creates a nested NCR. This NCR contains all the other columns for this navigation step, as well as all the columns generated by the subtree of navigations beneath it. For example:

-   -   self(a,--)—_(N)child(b,$b,$btable)—child(c,$c)         -   —child(d,$d)

The root (top-level) navigation may also be nested or unnested. In addition, the LMatch operator, like other operators, provides an additional column that names the its entire schema. The child(b) navigation is a nested navigation that results in a nested NCR, named $btable, in the result. This NCR will contain columns $b (because $b is a column on the child(b) navigation) and $c (because $c is a column on a navigation in child(b)'s subtree). FIG. 18 illustrates a final NCR structure.

A depth-first traversal of the match expression of an LMatch operator is used to construct the columns of the output NCR. As a result, the LMatch operator also defines an ordering of the columns as well as their structure and names.

When a navigation matches multiple times, then the results differ based on whether the navigation is nested. If the navigation is a nested navigation, then a nested NCR is created, which will contain the matches. But if the navigation is not nested, then the results are combined via a cross-product with all the other columns in the same table. So, if one <b> element contained multiple <c> elements, the $btable would contain the corresponding <b>-<c> pairs. Navigations that are not nested can be treated as a special case of nested navigations. Thus, an LMatch operator can be evaluated as if all navigations are nested. Then, for each navigation that is actually nested, a an LFlatten operation can be used to remove the table corresponding to the nesting.

A subtree column results in the entire XML subtree, tags and all, being returned as an atomic value. (This corresponds to the ELEMENT_AS notation in XML-QL.) The compiler transforms this column into a more complex LMatch expression that “pulls apart” the entire subtree contents and modifies the rest of the execution unit to reconstruct the result back into a subtree when needed. As a result, subtree columns exist initially, but they are replaced with more complex patterns. Before they are rewritten, the subtree columns are modeled in the NCR schema as a single, static column. After the rewrite, they begin with a table-valued column containing the nested results.

Advantages of the LMatch operator being a single, complex operation include:

-   -   1. When queries are generated for query languages which         themselves contain some form of matching operations, then         mapping onto those operations is enabled.     -   2. Certain optimizations that may be done on navigational         matching are better enabled by capturing succinctly the         navigation that is being done. In particular, reasoning about         substitution of a descendant relation with a union of paths, and         vice versa. Also, also reasoning about document order relations.     -   3. The LMatch operator combines two kinds of capabilities into a         single operator: navigational operations and composition of the         results into a complex structure (the NCR). This allows a         concise representation of a very common idiom.

The LMatch operator can be matched against a tree that represents an XML generator, rather than the actual XML document. For example,

-   -   The XML RDB Map can be interpreted as a generator of an XML         document from a relational database. Matching the LMatch         operator against an XML RDB Map is a fundamental step in         converting the XML query into SQL.     -   The Construct Program of a query can be interpreted as a         generator of an XML document from an NCR. Matching the LMatch         operator against a Construct Program is a fundamental step in         composing views.

The algorithm for matching against tree-structured XML generators is very similar to the algorithm for matching against XML input directly. One difference is that where matching against an XML document generates tuples of output, matching against a generator generally produces a Correspondence Tree, which encodes all the potential correspondence points between the nodes of the generator and the navigation steps of the LMatch.

An XML generator is a tree (actually, a forest suffices) where the nodes in the tree represent the generation of XML elements or attributes or their values, and arcs between nodes represent inclusion. For example:

-   -   element(“person”)—attribute(“ssn”)—value( )         -   —element(“name”)—value( )         -   —element(“address”)—value( )

The XML generator also indicates the arity of each arc. The values for arity are optional (0 or 1), singular (exactly one) and multiple (0 or more). If an arc is marked multiple, then the generator can generate more than one instance of the child node for each parent instance. In the above example, if the arc between “person” and “name” were marked multiple, then a person could have zero or more names. The arity of an arc is indicated by a subscript on the arc as shown in the following:

-   -   element(“person”)—_(S)attribute(“ssn”)—value( )         -   —_(M)element(“name”)—value( )         -   —_(O)element(“address”)—value( )

When no arity is indicated, singular is assumed. If it is not possible to derive arity information from the generator, then multiple is assumed, since it is the most general case.

The Correspondence Tree tracks which navigation steps in the LMatch operator correspond with which nodes in the XML generator. The Correspondence Tree would be isomorphic to the LMatch navigation graph except for one thing: any given navigation step might match against multiple nodes in the generator. The following is an example of an XML generator, an LMatch operator, and the corresponding Correspondence Tree:

The XML generator:

-   -   element(“person”)₁—attribute(“ssn”)₂—value( )₃         -   —element(“name”)₄—value( )₅         -   —element(“name”)₆—value( )₇

The LMatch:

-   -   selft(person)₁—child(ssn)₂         -   —child(name)₃

FIG. 19 illustrates the Correspondence Tree.

The subscripts on nodes in the generator and LMatch distinguish otherwise identical nodes when they appear in the Correspondence Tree. The Correspondence Tree is “read” as: “The root navigation has a single match, namely the element(“person”)₁ node of the XML generator. From this generator node, the next LMatch navigation, child(name)₃, is matched against two different generator nodes, and so on.

The Correspondence Tree is a bipartite graph. A bipartite graph is one in which nodes come in two different alternating types. In this case, the node types are called navigation nodes (which reference navigation steps, and are pictorially indicated with brachets [ ]) and choice nodes (which reference generator nodes, and are pictorially indicated with braces { } ). A bipartite graph is interpreted as having two different kinds of arcs, which are indicated by lines of different weights: light lines are choice arcs (arcs from navigation to choice nodes, choosing amongst multiple correspondences) and heavy lines are navigation arcs (arcs from choice to navigation nodes, following the navigation relationships in the LMatch operator).

A correspondence is a (navigation step, generator node) pair of a correpondence tree. A correspondence is derived from a choice node by including the navigation step from the parent. For example, the following subgraph of a correspondence tree yields the following correspondence:

-   -   subtree: [child(name)₃]—{S: element(“name”)₄}     -   correspondence: {child(name)₃, element(“name”)₄}

The following matching algorithm generates the Correspondence Tree, given an LMatch operator and an XML generator as input. The algorithm is a top-down recursion over the LMatch navigation graph.

The XML generator has the following operations:

XMLGenerator.root() → ordered list of GeneGeneratorNode GeneratorNode.type() → { “element” | “attribute” | “value” } GeneratorNode.name() → GName GeneratorNode.genChildren() → list of GeneratorNode GeneratorNode.arity(childNode) → { “S” | “M” | “O” }

In this example, the LMatch operator is limited to the following navigation types: root, child, self. The nested flag on LMatch navigation steps is irrelevant to matching. The LMatch operator provides the following pseudo code for accessing the match expression:

LMatch.root() → NavStep NavStep.type() → { “root” | “child” | “self” } NavStep.ea() → { “element” | “attribute” | “either” } NavStep.name() → NName NavStep.navChildren() → list of NavStep NavStep.optional() → boolean

There is also a function, nameMatch(GName, NName)→boolean, that returns true or false as the name from a generator node matches the name of an LMatch navigation. The Correspondence Tree provides the following operations:

CorrespondenceTree.root() → NavigationNode CorrespondenceTree.createRoot( NavStep ) NavigationNode.new( NavStep ) NavigationNode.navStep() → NavStep  // Model .navStep() . type () NavigationNode.type() → { “root” | “child” | “self” }  // And .navStep () .name () NavigationNode.name () → NName NavigationNode.choiceChildren() → list of ChoiceNode NavigationNode.addChoiceChild( ChoiceNode ) ChoiceNode.new( GeneratorNode, arity ) ChoiceNode.generatorNode() → GeneratorNode ChoiceNode.type() → { “element” | “attribute” | “value” } // ditto ChoiceNode.name() → GName   // ditto ChoiceNode.arity() → { “S” | “M” | “O” }   // ditto ChoiceNode.navChildren() → list of NavigationNode ChoiceNode.addNavChild( NavigationNode )

The following illustrates the BuildCorrespondence function that is invoked to build a Correspondence Tree for an XML generator and an LMatch operator:

// Assume a rooted LMatch match expression; // normalize the LMatch to make this true if necessary. BuildCorrespondence( XMLGenerator g, LMatch lm ) { // create the correspondence tree ct ← new CorrespondenceTree nn ← new NavigationNode( lm.root() ) // bootstrap the first level of expansion, // matching root against roots ct.addRoot( nn ) foreach( gn in g.getRoot() ) { cn ← new ChoiceNode( gn, “M” ) if ( addNavs( nn, cn ) ) nn←addChoiceChild( cn ) { return ct {

The form of the algorithm is mutual recursion between two functions, each of which extends the graph by one level, or fails to do so (because there is no match). The subroutines return boolean values indicating whether or not they were successful; this value is then used to determine whether or not to continue and whether or not to actually add nodes to the graph. The following is the pseudo code for the addNavs function:

// From a given corresponding navigation and choice node pair, // extend the choice node for each child navigation of the // navstep. boolean addNavs ( NavNode nn, ChoiceNode cn ) { foreach ( step in nn.navStep() .navChildren() ) { stepnavnode ← new NavigationNode( step ) success ← addChoices( cn, stepnavnode ) // if a navigation is optional, we include the navNode, // even if it failed (the navNode will have no choice children) if ( success || step.optional() ) cn←addNavChild( stepnavnode ) else // failure of a required navigation; abort return false { // if no required navigation failed, return true return true {

The following is the pseudo code for the addChoices function:

// Given a location in the generator and a requested // navigation, “follow” the navigation in the generator tree, // finding a new layer of correspondences. boolean addChoices( ChoiceNode cn, NavNode nn ) { success ← false foreach( gn in follow( cn.generatorNode(), nn.navStep() ) { choicenode ← new ChoiceNode( gn, cn.generatorNode() .arity(gn) ) thissuccess ← addNavs( nn, choicenode ) if ( thissuccess ) { nn←addChoiceChild( choicenode ) success ← true { { // return true if at least one choice worked out return success; {

The following is the pseudo code for the follow function:

// Implement the actual navigation; this would be extended // with more types of navigation as the LMatch is extended // (and would probably require the generator to support more // powerful navigations as well, at least parent()). List<GeneratorNode> follow( GeneratorNode gn, NavStep nav ) { List<GeneratorNOde> result ← (); switch( nav.type() ) { case “self” : if ( nameMatch( gn.name(), nav.name() ) result.add( gn ) case “child” : foreach ( gnkid in gn.genChildren() ) if ( (nav.ea() == “element” || nav.ea() == “either”) && gnkid.type() == “element” && nameMatch( gn.name(), nav.name() ) { result.add( gn ) else if ( (nav.ea() == “attribute” || nav.ea() == “either”) && gnkid.type() == “attribute” && nameMatch( gn.name(), nav.name() ) { result.add( gn ) case “root” : foreach ( r in xmlGenerator.root() ) // [1] if ( namematch( r.name(), nav.name() ) result.add( r ) { return result {

The BuildCorrespondence algorithm presented above does not match against actual XML data. However, an XML document may be considered a degenerate XML generators with singular-arity arcs and constant value nodes and and NCR is built rather than a Correspondence Tree. The relationship between a Correspondence Tree and an NCR is as follows:

-   -   The values for any particular navigation are the concatenation         of the values for each choice below that navigation; the result         is a list of data.     -   For nested navigations, the rows of the nested table are that         list of data.     -   For an unnested navigation that is at most singular, then the         list of data can contain only 0 or 1 rows. In this case, the NCR         column is essentially a field that is filled in the by value.     -   In general for unnested navigations, the list of data is “joined         against” the existing rows of the containing table. If the         navigation is optional, the join is an outer join, if required,         an inner join. If the list has multiple entries, the effect is a         cross product against the other contents of the table.

Because navigations can result in failure that propagates recursively upwards, matches to the leaves are evaluated before committing to any results. Alternatively, the LMatch operation could contain only optional navigations or only required navigations in cases where the data will be present. Similarly, it is possible to eliminate the need to handle joins or cross products by limiting the LMatch operator to only allow unnested navigations when the data is at most singular.

Two type of normalization that can be performed on LMatch operators are removal of (non-root) self navigations and removal of implicit cross-products. The normalized LMatch operator would consist only of a single root self navigation and following child navigations, where for each child navigation, nested=true. Alternatively, the normalization could cover either (nested=true) or (nested=false and optional=false and the child is known to exist in a strict 1:1 relationship with the parent). Additional normalizations, such as requiring optional=true on all nested child steps, may also possible.

To normalize the LMatch operator, additional operators are inserted to the Logical Extract Program to compensate for the changes to the LMatch operator. These logical operators include the LSelect, LFlatten, and LBox operators. The LSelect operator removes tuples from a table based on some condition. The LFlatten operator flattens a nested table within an NCR. The operator is applied to a single nested table, and the process of flattening removes that table. The LFlatten operator has a boolean parameter “outer” indicating whether the flattening operation should behave like an inner or left outer join—that is, if the nested table is empty, does flattening remove the containing row or not. The LBox operator serves to introduce an artificial level of nesting within a table.

A singular relationship between a child navigation and its parent navigation is identified by examining the XML schema of the data that the LMatch operates against. Initially, the matching algorithm has been run. After that, it can be determined, for each navigation step, which place(s) in the schema the LMatch operator could match. From that information, and from the cardinality information available in the schema, it can be identified whether the singular condition holds.

The first version of the algorithm generates an LMatch that contains a single, top-level self navigation and otherwise contains only child navigations. All navigations (including the self at the top) have nested=true. The resulting navigations may have optional=true or optional=false. The implementation can be styled in a bottom-up or top-down traversal, but note that in either case compensating operators are to be inserted at both the bottom and top of the chain.

The table below illustrates the various cases that can arise. The right-hand column has examples of the transformations. Here is a sample XML document this can be tested against:

<a>a1<b>b1</b> <c>c1<d>d1</d></c></a> <a>a2<b>b1</b><b>b2</b> <c>c1<d>d1</d></c> <c>c2<d>d2</d></c></a> <a>a3 <c>c1<d>d1</d></c> <c>c2<d>d2</d><d>d3</d></c></a> <a>a4<b>b1</b></a>

FIGS. 20–25 illustrate normalization. If the step is the root self step of the navigation tree, and has FIG. 20 nested = false, treat it the same way as a child step with nested = false (see below). If the step is a child step with nested = true, do nothing. If the step is a child step with nested = false, set nested = true, FIG. 20 autogenerate a table column for the new nested table, and add an LFlatten operator to compensate. The LFlatten target is set to the nested table that this step generates. If the child step has optional = true, the LFlatten must be an FIG. 21 “outer” LFlatten. If the step is a self step with optional = false, and nested = FIG. 22 false, remove the step, migrating the children of the self step to the parent. If the self step had columns on anything, add those columns to the parent step. If one of the migrated columns is a duplicate of an existing column of the parent step, use renaming to remove one of the columns from the entire LEP. If the step is a self step with optional = true and nested = true, FIG. 23 remove the step, migrating the children of the self step to the parent. If the self step had columns on anything other than its table column, add those columns to the parent step, unless it would clash with an existing column on the parent step. In that case, add an LDup operator to make a new copy of the self step's column. Determine the total set of columns that ‘belong to’ the self step (including the result of the LDup, if any), and insert an LBox operator to nest those columns, giving the result the original table name from the omitted self. The LBox operator is inserted after the LDup, if there is one, but before all other steps. If the step is a self step with optional = false and nested = true, FIG. 24 proceed as in the case above. Then add an LSlelect operator to test for emptiness of nested table created by the LBox. Unlike other operator additions, this LSelect operator must be added to the end of the chain of operators that have been added, so that it operates only after any flattenings have been done at deeper levels of nesting. If the step is a self step with optional = true and nested = false, proceed as in the case above, except instead of an LSelect step at the end, insert an LFlatten with outer = true.

In one embodiment, the following optimization may be applied. If an LBox is followed by the flattening of all its columns, the nested tables can be joined with a sequence of LJoin operators (as cross products) instead. This optimization could be performed either during this algorithm, or as a post-processing step. To illustrate, the last example above could be rewritten as shown in FIG. 25.

Alternatively, the normalization can be modified to state that only nested=true are added to child steps that can have multiple (or, possibly, optional) values. This normalization is may be easier for inputs to create NCR′ in which 1:1 elements are listed as flat columns of a row; any nesting on these columns may need to be added by an explicit LBox operator. In the case of a child step that has nested=false, and the step has been marked as singular without changing the value of the nested flag and without adding an LFlatten operation. The other steps do not change; in particular elision of a self step in the general case may result in adding an LBox, possibly followed by a LSelect or LFlatten operation. However, if all child navigations of a self navigation are singular, then the LBox and corresponding LFlatten can be omitted. The corresponding LSelect needs to be changed to a test on the NULL-ness of the columns, rather than a test on the emptiness of a nested table. This condition can be detected in a post-processing step, but it would require information from both the LMatch (the singularity of steps) and correlated information from the logical extraction program (the presence of LBox and LFlatten/LSelect); thus, this optimization may be implemented as an integral part of the recursive algorithm.

Various disclosed embodiments also relate to an apparatus such as a computer for performing the disclosed operations. The computer may include one or more processors and one or more memories. Still further, various disclosed embodiments relate to machine-readable media on which are stored program instructions for performing operations on a computer or computer system. The computer readable medium can be any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, magnetic tape, and optical data storage devices.

From the above description, it will be appreciated that although the specific embodiments of the technology have been described for purposes of illustration, various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except by the appended claims. 

1. A computer-readable medium storing thereon computer-readable instructions, the computer-readable medium storing thereon a data structure, the data structure comprising: a plurality of rows; a type column adapted for storing a row type for each of the plurality of rows of the data structure, each row type indicating a structure of the corresponding row, wherein the structure includes a set of columns, the row type being selectable from a set of row types such that the row type indicates an identity of each of the set of columns, thereby enabling the structure for each of the plurality of rows to vary based upon the row type for that row, wherein the type column is separate from the set of columns of the structure identified by each row type stored in the type column; computer-readable instructions for storing data retrieved from two or more data stores in the set of columns for the plurality of rows; and computer-readable instructions for storing a row type for each of the plurality of rows of the data structure in the type column, wherein the row type for a corresponding one of the plurality of rows indicates an identity of the set of columns of the structure storing data for the corresponding row; wherein the two or more data stores include a first data store and a second data store, wherein the first data store stores data in a different format from data stored in the second data store, thereby enabling data retrieved from data stores storing data in different formats to be stored in a single data structure.
 2. The computer-readable medium of claim 1 wherein the data structure is a nested conditional relation data structure.
 3. The computer-readable medium of claim 1 wherein at least two rows of the data structure contain different row types in the type column.
 4. The computer-readable medium of claim 1 wherein the row type for each of the plurality of rows identifies a schema for a type.
 5. The computer-readable medium as recited in claim 1, the data structure further comprising: a nested data structure in a column of one of the plurality of rows of the data structure, the nested data structure including sub-rows and a type sub-column, each of the sub-rows having an associated row type in the type sub-column, each row type in the type sub-column indicating a structure of the corresponding sub-row, wherein the structure of the corresponding sub-row includes a set of one or more sub-columns of the nested data structure associated with the corresponding sub-row, the row type in the type sub-column being selectable such that the sub-row type indicates an identity of each of the set of sub-columns, thereby enabling the set of sub-columns for each of the plurality of sub-rows to vary based upon the row type for that sub-row, wherein the type sub-column is separate from the set of sub-columns identified by the row types stored in the type sub-column.
 6. The computer-readable medium of claim 5 wherein a sub-column of one of the sub-rows of the nested data structure includes a further nested data structure.
 7. The computer-readable medium of claim 5 wherein at least two sub-rows of the data structure contain different row types in the type sub-column.
 8. The computer-readable medium as recited in claim 1, wherein the row types stored in the type column are not data elements and the columns identified by the row types are adapted for storing data elements.
 9. The computer-readable medium as recited in claim 1, wherein data is stored in the columns for each of the plurality of rows, thereby enabling the data to be retrieved from the columns indicated by the row type for that row.
 10. The computer-readable medium as recited in claim 9, further comprising: computer-readable instructions for retrieving data from one or more of the plurality of rows of the data structure, where the retrieved data is obtained from one or more of the columns indicated by the row type for that row.
 11. The computer-readable medium as recited in claim 1, further comprising: computer-readable instructions for retrieving data from one or more of the plurality of rows of the data structure, where the retrieved data is obtained from one or more of the columns indicated by the row type for that row.
 12. The computer-readable medium as recited in claim 1, further comprising: computer-readable instructions for receiving a query directed to multiple data stores including a first data store and a second data store, wherein the query is in the second format, the first data store is in the first format and the second data store is in the third format; computer-readable instructions for generating a first query directed to the first data store based on the first format using the received query and a mapping between the first format and the second format; computer-readable instructions for generating a second query directed to the second data store based on the third format using the received query and a mapping between the third format and the second format; computer-readable instructions for executing the first generated query based on the first format against the first data store in the first format to generate first data wherein the generated first data is stored in the data structure; and computer-readable instructions for executing the second generated query based on the third format against the second data store in the third format to generate second data wherein the generated second data is stored in the data structure.
 13. The computer-readable medium as recited in claim 1, wherein the row type indicates a type of record storing the corresponding row.
 14. The computer-readable medium as recited in claim 1, wherein the structure is a record and wherein the row types indicates a record type of the record.
 15. A computer-readable medium for storing data in a data structure, comprising: instructions for retrieving data from two or more data stores, wherein the two or more data stores include a first data store and a second data store, wherein the first data store stores data in a different format from data stored in the second data store; instructions for identifying a row type corresponding to data to be stored in each of the plurality of rows of the data structure, each row type indicating a structure of the corresponding row, wherein the structure includes a set of columns, the row type being selectable from a set of row types such that the row type indicates an identity of each of the set of columns, thereby enabling the set of columns for each of the plurality of rows to vary based upon the row type for that row; instructions for storing the row type for each of the plurality of rows of the data structure in a type column of the data structure, thereby enabling the structure for each of the plurality of rows to vary based upon the row type for that row, wherein the type column is separate from the set of columns identified by the row types stored in the type column; and instructions for storing the data retrieved from the two or more data stores in the set of columns for the plurality of rows, thereby enabling data retrieved from data stores storing data in different formats to be stored in a single data structure.
 16. The computer-readable medium of claim 15 wherein the data structure is a nested conditional relation data structure.
 17. The method as recited in claim 16, further comprising: converting first data from a first format to a nested conditional relation prior to storing the data; wherein storing the data includes storing the converted first data.
 18. The method as recited in claim 17, further comprising: converting second data from a second format to a nested conditional relation prior to storing the data; wherein storing the data further includes storing the converted second data.
 19. The computer-readable medium of claim 15 wherein at least two rows of the data structure contain different row types in the type column.
 20. The computer-readable medium of claim 15 wherein the row type for each of the plurality of rows identifies a schema for a type.
 21. The computer-readable medium of claim 15, further comprising: instructions for obtaining a query directed to multiple data stores including a first data store and a second data store, wherein the query is in the second format, the first data store is in the first format and the second data store is in the third format; instructions for generating a first query directed to the first data store based on the first format using the received query and a mapping between the first format and the second format; instructions for generating a second query directed to the second data store based on the third format using the received query and a mapping between the third format and the second format; instructions for executing the first generated query based on the first format against the first data store in the first format to generate first data wherein the generated first data is stored in the data structure; and instructions for executing the second generated query based on the third format against the second data store in the third format to generate second data wherein the generated second data is stored in the data structure.
 22. The computer-readable medium of claim 21, further comprising instructions for converting the data of the created data structure into data in the second format.
 23. The computer-readable medium of claim 21 wherein the second format is an XML format.
 24. The computer-readable medium as recited in claim 15, wherein the data structure further comprises: a nested data structure in a column of one of the plurality of rows of the data structure, the nested data structure including sub-rows and a type sub-column, each of the sub-rows having a row type stored in the type sub-column, each row type in the type sub-column indicating a structure of the corresponding sub-row, wherein the structure of the corresponding sub-row includes a set of one or more sub-columns of the nested data structure associated with the corresponding sub-row, the row type in the type sub-column being selectable such that the sub-row type indicates an identity of each of the set of sub-columns, thereby enabling the set of sub-columns for each of the plurality of sub-rows to vary based upon the row type for that sub-row, wherein the type sub-column is separate from the set of sub-columns identified by the row types stored in the type sub-column.
 25. The computer-readable medium of claim 24 wherein a sub-column of a sub-row of the nested data structure includes a further nested data structure.
 26. The computer-readable medium of claim 24 wherein at least two sub-rows of the nested data structure contain different row types in the type column.
 27. The computer-readable medium as recited in claim 15, wherein the row types stored in the type column are not data elements and the columns identified by the row types are adapted for storing data elements.
 28. The computer-readable medium as recited in claim 15, wherein the data stored in each of the columns is a primitive type or a nested conditional relation.
 29. The computer-readable medium as recited in claim 15, further comprising: instructions for retrieving data from one or more of the plurality of rows of the data structure, where the retrieved data is obtained from one or more of the columns indicated by the row type for that row.
 30. A computer system for storing data in a data structure having a plurality of rows, comprising: a processor; and a memory, at least one of the processor or the memory being for: retrieving data from two or more data stores, wherein the two or more data stores include a first data store and a second data store, wherein the first data store stores data in a different format from data stored in the second data store; identifying a row type corresponding to data to be stored in each of the plurality of rows of the data structure, each row type indicating a structure of the corresponding row, wherein the structure includes a set of columns, the row type being selectable from a set of row types such that the row type indicates an identity of each of the set of columns, thereby enabling the set of columns for each of the plurality of rows to vary based upon the row type for that row; storing the row type for each of the plurality of rows of the data structure in a type column of the data structure, thereby enabling the structure for each of the plurality of rows to vary based upon the row type for that row, wherein the type column is separate from the set of columns identified by the row types stored in the type column; and storing the data retrieved from the two or more data stores in the set of columns for the plurality of rows, thereby enabling data retrieved from data stores storing data in different formats to be stored in a single data structure.
 31. The computer system of claim 30 wherein the data structure is a nested conditional relation data structure.
 32. The computer system of claim 30 wherein at least two rows of the data structure contain different row types in the type column.
 33. The computer system of claim 30 wherein the row type for each of the plurality of rows identifies a schema for a type.
 34. The computer system of claim 30, at least one of the processor or the memory being for: providing a mapping of a first format to a second format; providing a mapping of a third format to the second format; receiving a query directed to multiple data stores including a first data store and a second data store, wherein the query is in the second format, the first data store is in the first format and the second data store is in the third format; generating a first query directed to the first data store based on the first format using the received query and the mapping between the first format and the second format; generating a second query directed to the second data store based on the third format using the received query and the mapping between the third format and the second format; executing the first generated query based on the first format against the first data store in the first format to generate first data wherein the generated first data is stored in the data structure; and executing the second generated query based on the third format against the second data store in the third format to generate second data wherein the generated second data is stored in the data structure.
 35. The computer system of claim 34 including converting the data of the created structure into data in the second format.
 36. The computer system of claim 34 wherein the second format is an XML format.
 37. The computer system as recited in claim 30, wherein the data structure further comprises: a nested data structure in a column of one of the plurality of rows of the data structure, the nested data structure including sub-rows and a type sub-column, each of the sub-rows having a row type stored in the type sub-column, each row type in the type sub-column indicating a structure of the corresponding sub-row, wherein the structure of the corresponding sub-row includes a set of one or more sub-columns of the nested data structure associated with the corresponding sub-row, the row type in the type sub-column being selectable such that the sub-row type indicates an identity of each of the set of sub-columns, thereby enabling the sub-columns for each of the plurality of sub-rows to vary based upon the row type for that sub-row, wherein the type sub-column is separate from the set of sub-columns identified by the row types stored in the type sub-column.
 38. The computer system of claim 37 wherein a sub-column of a one of the sub-rows of the nested data structure includes a further nested data structure.
 39. The computer system of claim 37 wherein at least two sub-rows of the nested data structure contain different row types in the type sub-column.
 40. The computer system as recited in claim 30, wherein the row types stored in the type column are not data elements and the columns identified by the row types are adapted for storing data elements. 